Generally, in order to manufacture a semiconductor integrated circuit, a variety of treatments; such as film formation treatments, etching treatments using a photolithography technique, oxidation treatments, diffusion treatments and modification treatments; are performed on semiconductor wafers such as silicon substrates or the like. In the photolithography technique, a photoresist is coated on the semiconductor wafers such as the silicon substrates or the like, and then is baked. Thereafter, a mask pattern is transcribed on the photoresist through an exposure process, i.e., by irradiating ultraviolet rays or the like on the photoresist through a photo mask. Finally, a resist pattern is formed by developing the photoresist.
The photoresist is a mixed liquid of, for example, photosensitizer, resins, solvent or the like. After the photoresist is coated on the semiconductor wafers, moisture or volatile components contained in the photoresist are evaporated by performing a pre-bake or a post-bake with respect to the photoresist coated semiconductor wafers. As a result, a thin film of the photoresist is baked as described above.
A vertical heat treatment apparatus is generally used for performing a post-bake as a sintering process because it is capable of baking a plurality of semiconductor wafers simultaneously.
The vertical heat treatment apparatus is configured to support a plurality of semiconductor wafers that have been coated by the photoresist and pre-baked in multi-levels in a vertically cylindrical processing container. The photoresist is baked by heating the processing container using a heater while supplying a large amount of inert gas such as a N2 gas into the processing container and by evacuating moisture or the volatile components generated from the photoresist together with the N2 gas. In the vertical heat treatment apparatus for example, the N2 gas is introduced into a lower region of the processing container, flows upward in the processing container, and is evacuated together with the volatile components.
However, the vertical heat treatment apparatus has a drawback that the lower region of the processing container is likely to become a low temperature region in a cold spot state, because it is difficult to transfer the heat generated by the heater to the lower region and a heat dissipation amount is large in the lower region. Further, vaporized gas including sensitizer components as well as pure volatile components is sometimes generated during the baking process. When the vaporized gas is brought into contact with the low temperature region, the vaporized gas is cooled and powdered or liquefied depositions causing particles adhere to the low temperature region. For example, when a polyimide resin is used as the photoresist, a tar-like liquid including carbon adheres to the low temperature region.